Synergic control in MIG welding and penetration control in TIG welding

Naseer-Ahmed, (1992) Synergic control in MIG welding and penetration control in TIG welding. Doctoral thesis, London Guildhall University.


Part 1: Synergic control in MIG welding.
Synergic controls (i.e. real time control methods) for both steady DC open arc and short circuiting arc operations have been developed in this study. These have been based on the generalised quadratic and linear ‘power-current' and linear ‘voltage-current’ equations which adequately describe the MIG welding operation. The controls are low coat electronic units which are added to a ‘transistor’ controlled power source. The units based on the ‘power-current’ concept can operate with the power source set in the constant current, constant voltage or any intermediate mode of output characteristics, and regulate only the steady DC open arc operation. The unit based on the ‘voltage-current’ concept operates with the power source set in nominally constant voltage mode, and regulates both steady DC open arc and short circuiting arc operations. These controls adjust the current, voltage or power automatically according to any operator-selected wire feed speed whether maintained constant at any level, varied gradually or modulated with any waveform to achieve ‘thermal pulsing’.

In addition, two approaches have been used to adapt the synergic control units to a lower cost, thyristor controlled power source which is more widely used in industry. In one approach, the generalised control equation used previously for the transistor controlled power source has been extended to take account of the output characteristics of the thyristor controlled power source. As an alternative, the control units themselves have been modified to allow for the different power source characteristics.

The control systems have been successfully demonstrated for producing sound welds in a wide range of welding applications, for both mechanised and manual welding techniques.

Part II: Penetration control in TIG welding
A 'backface' penetration control system developed at Liverpool University, based on a ‘video camera' instead of a ‘photodiode' as a sensor, has been evaluated at The Welding Institute. Essentially, the system controls the size of the weld pool, instead of an average level of radiation, by regulating pulse current period, by means of a fibre optics image guide/video camera/microcomputer based controller. The system has been found to be capable of controlling the weld bead penetration uniformly in stainless steel plates and joints, for constant as well as variable material thicknesses.

However, the system cannot be used with high frequency (HF) arc discharge, as required for automatic arc initiation, because the software and electronic components are corrupted.

A hard-wired control unit has therefore been developed at The Welding Institute to replace foe microcomputer based controller. This unit can be operated reliably with automatic HF arc initiation. Furthermore, it has been shown to control weld bead penetration in stainless steel plates and joints, for constant at well as variable material thicknesses.

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